This invention relates to a direction finding antenna system. More specifically, it relates to a receiver system in a Traffic/Alert Collision Avoidance System (TCAS) which is particularly suited for determining the bearing of a target aircraft relative to a protected aircraft.
The TCAS equipment located aboard a protected aircraft periodically transmits interrogation signals which are received by transponders located aboard other aircraft, hereinafter referred to as target aircraft, in the vicinity of the protected aircraft. Mode S altitude reporting transponders are preferred so that TCAS capabilities can be fully exploited. In reply to the interrogation signals, the target aircraft's transponder transmits a response signal. The TCAS equipment aboard the protected aircraft determines the range of the target aircraft in accordance with the round trip time between transmission of the interrogation signal and receipt of the response signal. In addition, if the target aircraft is equipped with a Mode S transponder, the protected aircraft can determine the altitude of the target aircraft from the content of the response signal.
Information in the response signal, as well as information derived from the response signal, is used by the TCAS equipment to determine whether there is a threat of collision between the protected aircraft and the target aircraft. The response from each target aircraft is processed individually to determine the degree of threat and then, if necessary, an appropriate advisory is issued by the TCAS equipment to the pilot of the protected aircraft to minimize the degree of threat.
The target aircraft s bearing is usually derived from the response signal by a multi-element antenna array, for example, by a four element antenna array and an associated receiver system. In a conventional four element antenna array, there are two pairs of antenna elements. The elements are preferably spaced equally about the circumference of the antenna array, that is at 90.degree. intervals. A first pair of elements, therefore, are aligned on an axis which is perpendicular to an axis on which the second pair of elements are aligned. Adjacent elements are those which are circumferentially spaced apart by 90.degree.. Opposite elements are those which are circumferentially spaced apart by 180.degree..
Conventionally, the bearing of the target aircraft is determined by measuring the phase difference of the response signal between opposite element pairs. If both pairs of opposite elements are used, for example, then the phase difference between one opposite element pair is K sin (.beta.) and the phase difference between the other opposite element pair is K cos (.beta.), where K is the maximum phase excursion of the response signal between the elements in the respective opposite pair, and .beta. is the angle of target signal incidence with respect to the axes connecting antenna elements within an element pair, and .beta. is the angle of target signal incidence with respect to the axes connecting antenna elements within an element pair. The actual bearing is then calculated as follows: EQU .beta.=tan.sup.-1 (K sin (.beta.)/K cos (.beta.))
Such a system functions properly as long as K is less than 180.degree.. When K is greater than or equal to 180.degree., however, an ambiguity exists as phase detectors in the receive system are unable to properly distinguish phase differences. In such cases, for example, the phase detector cannot differentiate between measured phase differences of 180.degree. and -180.degree.. Under these circumstances the bearing of the intruder aircraft, therefore, can not be determined with certainty. This ambiguity conflicts with a desire to utilize an antenna which has the greatest phase excursion between opposite elements in order to maximize the signal to noise ratio of the system.
Also, in a phase only measurement system, the phase error of the measurement means can not be ignored. For example, some of the receive system induced phase errors can not be corrected by factory calibration of system equipment since phase variations occur in the system components with varying temperature, age and other variables.